Sheathing uranium



Feb. 3, v.1959 E. w. coLBEcK 2,871,558..

SHEATHING URANIUM Filed July 2,41946 smc 'wmsARLS coLmgck d Irventqr States SHEATHING URANIUM This invention relates to a method of sheathing uranium which, at least in its purer forms, tends to deteriorate rapidly on contact with air, steam or water. Although it is possible to protect uranium when it has been rolled or extruded into the form of rods by drawing down on to these rods protective tubes, there is a wastage of up to 50% of the weight of the original cast metal in machining operations prior to and after extrusion. This wastage is due to the necessity of removing scale, oxides, `unsound metal etc. from the surface of the crude billets and again from the extruded rods.

An object of the present invention is to facilitate the working of uranium metal.

Another object is to facilitate the fabrication'of protectively-sheathed uranium bodies.

Another object is to provide an improved method of making composite rods of uranium sheathed in another metal.

According to the present invention sheathed uranium is made by casting molten uranium into a container of protective metal.

The sheathing operation may be in accordance with the invention, for the primary purpose of facilitating the working of a cast vbillet of uranium to reduce it, for example by extrusion or rolling, to rod or wire or other form of predetermined cross-sectional shape and dimensions, and such working may include heat-treatment in one or more stages for modifying the ductility or for other purpose. The sheathing is then an integral part of a uranium metal working process. In such cases, when the uranium metal has been brought to its desired final shape and size, the sheathing metal may be stripped oi by a chemical, electrolytic or mechanical method or by a combination of such methods.

The sheathing operation may however serve the further purpose of providing a protective skin upon the worked uranium body to be elective during its useful life.

Alternatively, the sheathing operation may be carried out using a thin-walled container of protective metal conforming to the desired final dimensions ofthe sheathed uranium body. In this case, working of the cast uranium is unnecessary (although heat treatment may be applied) and the casting of the uranium into the container constitutes a direct method of making a protectively-sheathed shaped uranium body. Previous methods of making such sheathed bodies have involved the working and machining to shape ot' the uranium body from an initiallyformed billet with various measures to protect the metal from deterioration followed by either the tting of the uranium Ibody into a pre-formed container with measures to obtain `close continuous contact or the surface treatment of the uranium body with the object of establishing a protective coating.

The choice of metal for the container depends upon the functions which the sheathing is required to have and upon the conditions which arise in the casting operaatent c l Q Where it is desired to reduce the diameter of the initial assembly of uranium cast in the container, the sheath metal needs to have a capacity for deformation at roll- 'ing temperature for example 600 or 800 C., approximately equal to that of uranium so that the sheathed product can be readily rolled or extruded. It is also desirable for certain purposes, including the use of the product in an energy-producing pile, that the metal shall have a high thermal conductivity. Moreover the coefficient of thermal expansion of the sheathing metal should preferably approximate to that of uranium in order that excessive stress shall not arise between the sheath and the core and that cavities shall not be formed between- Convenient metals which have usefully favourable properties are for example copper and its alloys, alumi-l nium and austenitic chromium-nickel stainless steel. Of

these, aluminum while less convenient than copper forl metal working has a reasonably low capture cross-section.

In one method of carrying out this invention, we employ uranium metal of 99.9% purity. This metal is melted in a crucible, e. g. of graphite or thoria, conveniently having a means for bottom pouring, and means for avoiding contact with air is employed such as a flux, or enclosure and evacuation of the apparatus. The molten metaly is then poured into a previously prepared sheath of a suitable metal and allowed to solidify. The cornposite billet may then be rolled down into rods of say VW-1%" diameter which have on their surfaces a thin lm of say gf-Ma thickness.

It is clear that the thickness of the protecting metal sheath into which the molten metal is initially cast can be chosen so as to provide any desired thickness of protective skin in the nished bars.

In another method embodying the invention uranium is cast directly into thin-walled tubular containers which are thereafter closed and sealed to form composite rods walled cylindrical copper mould 3 split longitudinally to facilitate assembly. A graphite crown ring 4 mounted over the open end of the cannister and mould and serves tirstly as a funnel through which molten uranium is poured to lill the cannister and secondly as an upper boundary for the resultant casting. A vent 5 in the crown ring allows air to escape from the cannister during the casting operation.

Because of the rapidity with which the molten metal cools, the exposed surface of the casting tends to be irregular. If the crown ring be omitted, the uranium metal surface presents an irregular peak as indicated in Figure 2. The effect of the crown ring is to localise this peak so that it may more readily be removed by drilling or milling to leave a flat surface ready for a subsequent can-closing operation.

aanwas Various other methods may be employed to reach a level surface. For example the irregular surface result* ing from casting with or without the use of the crown ring, may be drilled to present a countersunk or other uniform surface over which is placed an aluminum disc of similar contour and with a flat upper face. Or the irregular surface may be covered with packing material, for example a brazing flux consisting of aluminum-silicon alloy as shown at 6 in Figure 2. In this figure the cast uranium 7 is shown in a cannister 8 having a closure disc 9.

The closing of the cannister is then effected in any of the Ways already proposed for sealing previously-machined uranium rods in thin-walled cannisters or in other convenient manner affording` a gas-tight seal. For example a closure disc of aluminium may be applied and the cannisterV Wall turned over and welded upon it 01- the closure disc may be flanged as shown, in Figure 2 and the seal completed by a peripheral weld between the edges of the cannister and the disc.

The process details may be modified to yield a sheathed rod of the section shown in Figure 3. After the casting operation the end of the composite rod, including the end of the cannister 8a, is cut back to present a flat surface over which a lid 9a is fitted and brazed or welded in position.

I claim:

A method of forming a conveniently handled corrosion resistant uranium article, comprising pouring molten uranium into an open-ended corrosion resistant metal container in which the metal is selected from the group consisting of copper and its alloys, aluminum, and austenitic-nickel stainless steel, covering the exposed surface of the cast uranium with a metallic packing material,

sealing the container in contact with substantially the entire exposed surface of said packing material, and then mechanically working said article to reduce the cross section thereof.

References Cited in the le of this patent UNITED STATES PATENTS Re. 2,570 Thompson Apr. 16, 1867 Re. 2,571 Thompson Apr. 16, 1867 52,465 Taylor Feb. 6, 1886 139,955 Hall June 17, 1873 338,849 Lorenz Mar. 30, 1886 666,452 Zucker Jan. 22, 1901 909,924 Monnot Jan. 19, 1909 1,162,339 Coolidge Nov. 30, 1915 1,568,685 Moore Ian. 5, 1926 1,597,189 Gero Aug. 24, 1926 1,648,962 Rentschler et al Nov. 15, 1927 1,733,744 Marden et al. Oct. 29, 1929 FOREIGN PATENTS 861,390 France Feb. 7, 1941 233,011 Switzerland Oct, 2, 1944 OTHER REFERENCES Driggs et al.: Preparation of Metal Powders by Electrolysis of Fused Salts I-Ductile Uranium, Ind. and Eng. Chem., vol. 22, No. 5, pp. 516-519, May 1930.

Business Week, pp. 57-64, Sept. 1.945.

Smyth: Atomic Energy for Military Purposes, p. 103, Aug. 1945. Copy may be purchased from Supt. of Doc., Wash. 25, D. C.

Kelly et al.: Physical Review 73, 1135-9 (1948). 

